1. Field of the Invention
The present invention relates to an illuminating device for a microscope with a lighting control circuit and, more particularly, to said illuminating device with a lighting control circuit which controls the commercial current by changing the firing angle.
2. Description of the Prior Art
For many of lighting devices for microscopes, SCR, triac, etc., are used in the lighting control circuits. That is, the firing angle of the input pulse to the gate of the SCR or triac is changed in order to control the current from the commercial power source, the current taken out being thus controlled is supplied to the primary winding of a transformer and the light-source lamp of the illuminating device is lit by the output from the secondary winding of said transformer. Such illuminating device provided with the lighting control circuit containing the SCR or triac in which the firing angle is changed has a disadvantage that a whiltling noise offensive to the ear is caused continuously by vibration of the transformer and of the filament of the source lamp. Especially when observing an object by a microscope at a quiet place, this noise is heard very loud and, for a sensitive person, this noise disturbs the observing work by the microscope. This kind of noise is inherent to an illuminating device with a lighting control circuit which controls the firing angle and it is very difficult to eliminate it.
To reduce said noise, a soundproofing or vibration-proofing device is sometimes provided to the illuminating device. In those methods, however, it is impossible to prevent the noise satisfactorily and, moreover, the illuminating device becomes large and it becomes not suitable for using as the illuminating device for a microscope.